General Information
Base Loading Control
Algorithm:
This feature allows an external controller to directly modulate the capacity of the chiller. It is typically used in applications where virtually infinite sources of evaporator load and condenser capacity are available and it is desirable to control the loading of the chiller. Two examples are industrial process applications and cogeneration plants. Industrial process applications might use this feature to impose a specific load on the facility’s elecrical system. Cogeneration plants might use this feature to balance the system’s heating, cooling and electrical generation.
All chiller safeties and adaptive control functions are in full effect when Base Loading control is enabled. If the chiller approaches full current, the evaporator temperature drops too low, or the condenser pressure rises too high, Tracer CH530 Adaptive Control logic limits the loading of the chiller to prevent the chiller from shutting down on a safety limit. These limits may prevent the chiller from reaching the load requested by the Base Loading signal.
Base Loading Control is basically a variation of the current limit algorithm. During base loading, the leaving water control algorithm provides a load command every 5 seconds. The current limit routine may limit the loading when the current is below setpoint. When the current is within the deadband of the setpoint the current limit algorithm holds against this loading command.
If the current exceeds the setpoint, the current limit algorithm unloads. The “Capacity Limited By High Current” message normally displayed while the current limit routine is active is suppressed while base loading.
Base loading can occur via Tracer, External signal, or front panel.
Tracer Base Loading: Current Setpoint Range:
(20 - 100) percent RLA
Requires Tracer and Optional Tracer Communications Module (LLID)
The Tracer commands the chiller to enter the base load mode by sending the base load mode request. If the chiller is not running, it will start regardless of the differential to start (either chilled water or hot water). If the chiller is already running, it will continue to run regardless of the differential to stop (either chilled water or hot water), using the base load control algorithm. While the unit is running in base loading, it will report that status back to the Tracer by setting “Base Load Status = true” in the Tracer Status Byte. When the Tracer removes the base load mode request (sets the bit to 0). The unit will continue to run, using the normal chilled or hot water control algorithm, and will turn off, only when the differential to stop has been satisfied.
External Base Loading: Current Setpoint Range: (20 - 100) percent RLA
The UCP accepts 2 inputs to work with external base loading. The binary input is at 1A18 Terminals
input, an analog input, is at 1A17 terminals J2 – 1 and 3 (Ground) which sets the external base loading setpoint, and can be controlled by either a
An alternative and less radical approach to Base Loading indirectly controls chiller capacity. Artifically load the chiller by setting the chilled water setpoint lower than it is capable of achieving. Then, modify the chiller’s load by adjusting the current limit setpoint. This method provides greater safety and control stability in the operation of the chiller because it has the advantage of leaving the chilled water temperature control logic in effect. The chilled water temperature control logic responds quicker to dramatic system changes, and can limit the chiller loading prior to reaching an Adaptive Control limit point.
24 |